RF amplifiers are used in a variety of applications, such as in power amplifiers of wireless transmitters. In some applications it is desirable to control the gain of an RF amplifier, e.g., maintain the gain at a constant level and compensate for gain variations.
Several techniques for controlling amplifier gain are known in the art. For example, U.S. Pat. No. 7,626,467, whose disclosure is incorporated herein by reference, describes an Automatic Level Control (ALC) technique to compensate for variations in the gain of an RF amplifier. The gain of the RF amplifier is controlled as a function of the output of a Voltage Controlled Oscillator (VCO) circuit. The VCO typically includes a buffer amplifier with a structure similar to that of the RF amplifier. By tracking changes in the output of the VCO buffer amplifier, an ALC input to the RF amplifier is adjusted to compensate for process- and temperature-based variations in amplifier gain.
U.S. Pat. No. 6,710,649, whose disclosure is incorporated herein by reference, describes a power amplifier module that includes a plurality of amplifier stages. Each amplifier stage includes a reference amplifier for emulating the operation of the amplifier. The current flowing to the base of a bipolar transistor that forms each reference amplifier depending on an input power level is detected, amplified and supplied as base current of the transistor of the corresponding amplifier.